Non-Hydrogen-Bonded Secondary Structure in β-Peptides:  Evidence from Circular Dichroism of (S)-Pyrrolidine-3- carboxylic Acid Oligomers and (S)-Nipecotic Acid Oligomers

Homooligomers of β-amino acids (S)-3-pyrrolidine-3-carboxylic acid (PCA) and (S)-nipecotic acid (Nip) were studied by circular dichroism (CD) in methanol. In each series, a profound change in the far-UV CD spectrum was observed from monomer to tetramer, but little change was observed from tetramer to hexamer. A comparable pattern is observed in the CD spectra of short proline oligomers. We conclude that both PCA and Nip oligomers with ≥ four residues adopt a characteristic secondary structure.